1. Field of the Invention
The invention disclosed herein relates to well logging tools and, in particular, to an improved light source and light detector for well logging tools.
2. Description of the Related Art
In exploration for hydrocarbons, it is important to male accurate measurements of properties of geologic formations. In particular, it is important to determine the various properties with a high degree of accuracy so that drilling resources are used efficiently.
Generally, oil and gas are accessed by drilling a borehole into the subsurface of the earth. The borehole also provides access for talking measurements of the geologic formations.
Well logging is a technique used to take measurements of the geologic formations from the boreholes. In one embodiment, a “logging tool” is lowered on the end of a wireline into the borehole. The logging tool sends data via the wireline to the surface for recording. Output from the logging tool comes in various forms and may be referred to as a “log.” Many types of measurements are made to obtain information about the geologic formations. One type of measurement involves determining gravitational acceleration or gravity.
Measurements of gravity can be used to determine information related to the mass of a surrounding formation. For example, measurements of gravity can be used to measure depletion of oil in the surrounding formation as water replaces the oil. When water replaces oil in the formation, the mass of the formation and, therefore, a gravitational force exerted by the formation will increase because water is denser than oil.
Measurements of gravity can also be used to determine true vertical depth in the borehole. The true vertical depth is important to know because borehole depth is a common factor among various logs. The various logs may be viewed side-by-side to form a composite picture of the geologic formations. Even small errors in determining the borehole depth can corrupt logging data. Horizontal deviations of the borehole, which can corrupt the logging data, can be accounted for by determining the true vertical depth using gravitational measurements.
A logging tool used for measuring gravity may employ a gravimeter that relates changes in gravitational acceleration to changes in light. This type of gravimeter requires a light source and a light detector. In order to obtain accurate measurements, it is important for the light source and the light detector to operate in a stable manner in the environment of the borehole.
In general, temperature in the borehole increases with increasing depth. In some instances, the temperature can be as high as 260° C. Additionally, the light source and the light detector may be subject to shocks of acceleration while traversing the borehole. To survive the rigors of the borehole environment, the light source and the light detector can be built using solid state technology. For example, the light source may include a laser diode and the light detector may include a photodiode.
Accurate measurements usually require that the wavelength of light emitted from the laser diode not shift more than ten ppm. In addition, the intensity of the light emitted from the laser diode should remain constant. With respect to the photodiode, the output should also remain stable for a stable input of light. With increasing temperature, the intensity of the light emitted from a laser diode decreases and the wavelength of the light increases to longer wavelengths. At high enough temperatures, such as temperatures within the borehole, most conventional laser diodes stop working. Providing stable laser light wavelength generally requires that the temperature of the conventional laser diode be maintained to within 0.001° C. Maintaining temperatures with this accuracy can be difficult in the borehole environment.
Therefore, what are needed are a light source and a light detector that can operate throughout a range of high temperatures and require less-stringent temperature control.